The most general technique for high-resolution imaging using interferometric methods is to partially fill the entrance pupil of the optical system with a two-dimensional array of collectors, and then combine their signals coherently such that an image of the source is constructed. In this way, the diffraction limited resolution of the full-sized entrance pupil can be approached or equalled. This is the approach used with the Very Large Array in Socorro, New Mexico, and with other image-forming radio interferometers used in the field of radio astronomy. In other environments such as space-borne applications, this approach would require a two-dimensional support structure spanning the desired entrance pupil in order to position the various collectors. This structure would have to be extremely rigid, or, alternatively, the positions of the collectors would have to be monitored continuously using, for instance, a network of auxiliary interferometers. These positional measurements would be used to correct the placement and orientation of the collectors, or to compensate for the optical path length errors from each collector. These requirements would be particularly stringent for instruments working at shorter wavelengths, such as the visible range. In any case, the accuracy and rigidity of the support structure would be compromised by the fact that, for any sizable entrance pupil, the structure would have to be unfolded or erected after being launched into orbit.
The problem is pronounced in space applications where the optical system must typically be packaged, at least during transport, in a generally elongated cylindrical volume from which the telescope is erected once orbit is achieved. The difficulty is in obtaining an accurate, large aperture telescope which can be packaged and transported in a much smaller volume than normally required for the full aperture configuration.